January 15, 2007

Many times over, we at the World Climate Report have underscored the popular idea that Earth’s frozen realm, or the cryosphere, serves as a monitor of regional climate variability and global climate change. This idea is combined with evidence and theory that a large degree of climate warming has and will occur in the high latitudes of the Northern Hemisphere. Climate variables that have been historically studied the most include air temperature, snow cover, and glacier characteristics, but in recent decades, sea ice data have been mined for use in climate change research.

While many studies have focused on trends in the characteristics of sea ice in the Northern Hemisphere over the last several decades, the ice cover of Canada’s Hudson Bay region has been almost entirely ignored. Evidence of an insignificant decrease in the spatial extent of the Hudson Bay sea ice (Parkinson et al. 1999) and an indication of earlier spring break-up and later fall freeze-up (Gagnon and Gough 2005a) generally support projections by general circulation models (Gagnon and Gough 2005b). However, trends in the thickness of Hudson Bay sea ice have not been examined in search of evidence of recent climate change…until recently. In the October 2006 issue of Climate Research, Alexandre Gagnon of the University of Liverpool (United Kingdom) and William Gough of the University of Toronto Scarborough (Canada) reported on trends in Hudson Bay sea ice thickness using data from the Canadian Ice Service.

Gagnon and Gough characterize the sea ice on Hudson Bay as going through a complete cryogenic cycle each year, whereby freeze-up typically occurs in October and November, the ice reaches peak thickness between March and May, and the region is ice-free beginning in early August. Hudson Bay is usually completely covered by ice from January to May. Only on rare occasions does ice in a small portion of the region survive the summer season.

Gagnon and Gough explain that the advection of heat in and out of Hudson Bay is negligible, and therefore the Bay acts like a closed ocean body. Thin ice associated with a warmer climate would permit greater conductive transfer of heat to the overlying atmosphere, thereby generating a positive feedback to the warm climate. Furthermore, the annual melting of the sea ice is hydrologically important, as it plays the largest role in the freshwater budget of Hudson Bay. The conclusion is that accounting for the sea ice cover of the Bay is necessary for regional climate change impact assessments. Gagnon and Gough also point out that most of the differences among the major general circulation models in their projections of climate in the Hudson Bay region were partially caused by changes to the seasonality of ice cover (Gagnon and Gough 2005b).

Data records of ice thickness and depth of the snow on the ice were extracted by Gagnon and Gough for 7 sea ice and 6 lake ice measuring stations within the Hudson Bay region for periods of as long as 1958 through 2003 and as short as 1972 through 1995. The researchers complimented the ice/snow data with seasonal temperature data from 17 weather stations in the region. Significant trends toward an increase in the annual maximum ice thickness were found at three sea ice stations and one lake ice station (E.g., Figure 1). Additionally, statistically significant trends toward an earlier occurrence of annual maximum ice thickness were found at three sea ice stations, two of which are not among the stations exhibiting a significant increase in ice thickness. In total, 5 of the 7 sea ice stations show either significant trends of increasing maximum ice thickness, earlier occurrence of the maximum thickness, or both. In contrast, only one station, a lake ice station, evidenced a significant thinning trend.

Figure 1. Annual variation in maximum ice thickness at three sample stations in the Hudson Bay region. The bold flat lines are the linear trend lines, the equations for which are noted. (Taken from Gagnon
and Gough, 2006)

Gagnon and Gough next turned their attention to complementary data, which resulted in mixed signals of climate change depending on the variable, time period, and location. In analyzing regional air temperature across the Hudson Bay region they conclude that temperatures changed over the second half of 20th century in much the same pattern as has been portrayed for the greater Northern Hemisphere: a peak in warming during the 1950s, followed by cooling in the 1960s and into the 1970s, and a renewed warming over the last couple of decades. Snow depth data from the ice thickness stations reveal very little trend over the periods of study, as only two stations from the 13 studied showed a significant trend – in each of these cases the trend was toward a thinner snow pack. Ice data from two of the stations showed a significant trend toward earlier freeze-up, with no significant trends at the remaining stations. This was contrasted by significant trends of an earlier break-up at 6 of the 13 stations.

From their analyses, Gagnon and Gough were able to construct two primary findings. First, they found “an asymmetry in temporal trends of landfast ice thickness; statistically significant thickening of the ice cover over time was detected on the western side of Hudson Bay, while a slight thinning lacking statistical significance was observed on the eastern side.” Second, they indicate that “this asymmetry is related to the variability of air temperature, snow depth, and the dates of ice freeze-up and break-up. Increasing maximum ice thickness at a number of stations is correlated to earlier freeze-up due to negative temperature trends in autumn. Nevertheless, changes in maximum ice thickness were reciprocal to the variability in the amount of snow covering the ground.”

Gagnon and Gough make it a point to note that their results “are in contrast to the projections from general circulation models (GCMs), and to the reduction in sea-ice extent and thickness observed in other regions of the Arctic.” Such contradictions are becoming more commonplace as new or lengthening data sets from the Arctic become available. Gagnon and Gough suggest that these types of contradictions “must be addressed in regional climate change impact assessments,” and we add that they must be addressed in any debate over climate change and its impacts.

References:

Gagnon A.S. and W.A. Gough. 2005a. Trends and variability in the dates of ice freeze-up
and break-up over Hudson Bay and James Bay. Arctic, 58, 370–382.